1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor laser apparatus which is constructed by bonding a semiconductor laser device to a base.
2. Description of the Related Art
FIG. 11 is a front view schematically showing a structure of a semiconductor laser apparatus 1 produced by a method for manufacturing a semiconductor laser apparatus according to the first prior art. FIG. 12 is a bottom view showing a semiconductor laser device 2 and a first bonding layer 3 of the semiconductor laser apparatus 1, as seen from a heat sink 4 side. According to the method for manufacturing the semiconductor laser apparatus according to the prior art practice, the semiconductor laser device 2 is bonded, with the first bonding layer 3 and a second bonding layer 5 made of a brazing filler material, to the heat sink 4 acting as a base.
The first bonding layer 3 is formed beforehand over the entire mount surface portion of the semiconductor laser device 2. The second bonding layer 5 is formed beforehand on the mount surface portion of the heat sink 4. The semiconductor laser device 2 is emplaced on the heat sink 4 after positioning. Subsequent to the emplacement of the semiconductor laser device 2 on the heat sink 4, the first and second bonding layers 3 and 5 are heated in a mutually pressed state. At this time, only the second bonding layer 5 is melted. Then, the second bonding layer 5 is cooled down, together with the first bonding layer 3, and is thereby allowed to set. Thereupon, the semiconductor laser device 2 is bonded to the heat sink 4 (refer to for example Japanese Unexamined Patent Publication JP-A 1-259587).
In the semiconductor laser device 2 thus bonded to the heat sink 4, with the setting of the molten second bonding layer 5, a stress is caused due to the difference in coefficient of thermal expansion between the semiconductor laser device 2 and the second bonding layer 5. This leads to occurrence of strain. When the strain appears in a light-emitting area 6 of the semiconductor laser device 2, the characteristics of the semiconductor laser device 2, specifically, for example the wavelength of the laser light emitted from the semiconductor laser device 2 is undesirably varied. Moreover, the strain adversely affects the service life of the semiconductor laser device 2.
FIG. 13 is a view showing the state of the stress occurring within the semiconductor laser device 2, as observed by a photoelasticity method. According to the photoelasticity method, that part of the semiconductor laser device 2 which is subjected to a stress exhibits a different birefringent property than the rest stress-free part thereof. By letting polarized infrared light incident on the semiconductor laser device 2, only the part under the stress is observed as a fringe resulting from interference of light. When polarized infrared light was incident on the semiconductor laser device 2 bonded to the heat sink 4, an interference fringe was observed in a shaded portion 7 depicted in FIG. 13. The shaded portion 7 includes part of the light-emitting area 6. This means that the light-emitting area 6 is also subjected to the stress.
To reduce the strain occurring in the light-emitting area, the second prior art proposes the following method. Firstly, the second bonding layer, which is formed on the heat sink, is patternized to create a void. Then, the semiconductor laser device is bonded to the heat sink, with its light-emitting area placed so as to coincide with the void of the second bonding layer (refer to for example Japanese Unexamined Patent Publication JP-A 63-308991).
According to this prior art practice, the light-emitting area of the semiconductor laser device and the void of the second bonding layer need to be positioned with high accuracy. Thus, the process is inevitably reduced in pace, resulting in the manufacturing cost being increased. Moreover, an expensive manufacturing apparatus capable of achieving accurate positioning is required. Another disadvantage is that, since the second bonding layer melts under the condition that the first and second bonding layers are pressed against each other, the molten second bonding layer is spread out in a direction perpendicular to the thickness direction. In addition to that, part of the second bonding layer is spread out over an unwanted area. That is, with the prior art practice, the strain occurring in the light-emitting area cannot be effectively reduced with ease.
FIG. 14 is a view schematically showing the structure of another semiconductor laser apparatus 8 produced by a method for manufacturing a semiconductor laser apparatus analogous to the method for manufacturing the semiconductor laser apparatus 1 shown in FIG. 11. FIG. 15 is a view showing the state of the stress occurring within the semiconductor laser devices 2a and 2b, as observed by the photoelasticity method. This prior-art semiconductor laser apparatus 8 is designed for use in an optical pickup, for example.
In the semiconductor laser apparatus 8, two pieces of semiconductor laser devices 2a and 2b are respectively bonded, with the first bonding layer 3a, 3b and the second bonding layer 5a, 5b, to a single heat sink 4 in a manner as described previously. The semiconductor laser device 2a, 2b has, in a portion 10a, 10b close to its one end face portion 9a, 9b, a light-emitting area 6a, 6b. 
When polarized infrared light was incident on the semiconductor laser device 2a, 2b bonded to the heat sink 4, an interference fringe was observed in a shaded portion 7a, 7b depicted in FIG. 15. The shaded portion 7a, 7b includes the light-emitting area 6a, 6b. This means that the light-emitting area 6a, 6b is also subjected to a stress. Moreover, it has been found that the portion 10a, 10b close to one end face portion 9a, 9b, in which the light-emitting area 6a, 6b is disposed, is subjected to a particularly heavy stress.
In the semiconductor laser apparatus 8 produced by the prior-art semiconductor laser apparatus manufacturing method, the light-emitting area 6a, 6b is disposed in the portion 10a, 10b close to one end face portion 9a, 9b of the semiconductor laser device 2a, 2b. In such a structure, the light-emitting area 6a, 6b suffers from appreciable strain.